The present invention relates to a technical field of display devices and, more particularly, to a display device such as a liquid crystal display that is equipped with a polarized light separator such as a polarizer or a reflective polarizer and that can be used as a reflective type in which external light is reflected to perform display and also as a transmissive type in which the light of a light source is transmitted to perform display, a driving method for the same, and electronic equipment such as a portable telephone, a watch, and a portable information terminal that employ the same.
Hitherto, in the case of a reflective type display device that makes use of external light to carry out display, the display becomes difficult to view as the quantity of light decreases in a dark place. On the other hand, a transmissive type display device that makes use of a backlight or other light source to carry out display consumes more electric power because of the light source regardless of whether the location where it is placed is bright or dark, and it is therefore not suited for a portable display device or the like operated on batteries in particular. Hence, a transflective type display device that can be used as the reflective type and also as the transmissive type is adapted to reflect external light entering through a display screen by a semi-reflective film provided therein while controlling the quantity of light outgoing from the display screen for each pixel by employing an optical element such as a liquid crystal or a polarized light separator disposed on an optical path thereof whereby to perform reflective display mainly for a bright place. On the other hand, mainly for a dark place, the transflective type display device is adapted to apply light source light by employing a built-in light source such as a backlight from the rear side of the foregoing semi-reflective film while controlling the quantity of light outgoing from the display screen for each pixel by employing an optical element such as a liquid crystal or a polarized light separator thereby to perform transmissive display.
A conventional liquid crystal display device that utilizes a variable transmission polarization axis optical element such as a TN (Twisted Nematic) liquid crystal, STN (Super-Twisted Nematic) liquid crystal or the like in which the polarization axis of transmitted light is rotated employs a structure wherein the variable transmission polarization axis optical element is sandwiched between two polarizers. The polarizers, which is an example of a polarized light separator, effects polarization by absorbing polarized light components in a different direction from that of a particular polarization axis direction from incident light, leading to a poor utilization factor of light. Especially in the case of the liquid crystal display device that can be used as the reflective type and also as the transmissive type described above, since light is reflected by the semi-reflective film for the reflective display, the utilization factor of light is worse. This poses a problem of dark display when the reflective display is carried out.
Referring to FIG. 33, a conventional transflective type display device employing a TN liquid crystal panel as the variable transmission polarization axis means will be described. FIG. 33 is a sectional view showing the conventional transflective type display device.
In FIG. 33, the display device is provided with an upper polarizer 205, an upper glass substrate 206, a TN liquid crystal layer that includes a voltage applied region 207 and a voltage non-applied region 208, a lower glass plate 209, a lower polarizer 210, a transflective plate 211, and a light source 212. As the transflective plate 211, an Al (aluminum) plate formed to be thin, for example, is used. Alternatively, the transflective plate 211 may be configured by providing a reflective plate with an opening. In FIG. 33, the respective components are shown as if they were separated for the sake of clarity; however, they are actually disposed in close contact to each other. Further, it is assumed that the upper polarizer 205 and the lower polarizer 210 are arranged such that the transmissive polarization axes are mutually orthogonalized to effect display in a normally white mode.
First, white display in the reflective display will be discussed. A light indicated on an optical path 201 is converted by the upper polarizer 205 into a linearly polarized light, which is directed parallel to a paper surface, twisted by 90 degrees in the direction of polarization in the voltage non-applied region 208 of the TN liquid crystal layer into a linearly polarized light perpendicular to the paper surface, transmitted as the linearly polarized light perpendicular to the paper surface through the lower polarizer 210, and reflected by the transflective plate 211, a part thereof being transmitted. The reflected light is transmitted again through the lower polarizer 210 as the linearly polarized light perpendicular to the paper surface, and twisted in the voltage non-applied region 208 of the TN liquid crystal layer by 90 degrees in the direction of polarization to become a linearly polarized light parallel to the paper surface, then it exits the upper polarizer 205. Thus, the white display is effected when no voltage is applied. In contrast to this, a light indicated by an optical path 203 is converted by the upper polarizer 205 into a linearly polarized light ray parallel to the paper surface, transmitted as the linearly polarized light parallel to the paper surface without being changed in the direction of polarization in the voltage applied region 207 of the TN liquid crystal layer, and absorbed by the lower polarizer 210, thus effecting black display.
The white display and the black display in the case of a transmissive display operation will now be described. A part of a light that is emitted from a light source 212 and that is indicated on an optical path 202 is transmitted through a transflective plate 211, converted on the lower polarizer 210 into a linearly polarized light perpendicular to the paper surface, twisted by 90 degrees in the direction of polarization in the voltage non-applied region of the TN liquid crystal layer to become a linearly polarized light parallel to the paper surface, and transmitted through the upper polarizer 205 as the linearly polarized light parallel to the paper surface, thus effecting the white display. In contrast to this, a part of a light that is emitted from the light source 212 and indicated on an optical path 204 is transmitted through the transflective plate 211, converted by the lower polarizer 210 into a linearly polarized light perpendicular to the paper surface, transmitted without being changed in the direction of polarization in the voltage applied region 207 of the TN liquid crystal layer, and absorbed by the upper polarizer 205, thus effecting black display.
As set forth above, the upper polarizer 205 and the lower polarizer 210 are both polarized light separators that involve the absorption; therefore, especially in the case of the reflective display, the light is partly absorbed when it is transmitted twice through the upper polarizer 205 and the lower polarizer 210. Furthermore, some of the light is transmitted through the transflective plate 211 to the light source 212 side and therefore not used for the display. As a result, the conventional transflective type liquid crystal display device has been posing a problem of a poor light utilization factor that causes a dark display screen especially in the case of the reflective display mode.
To solve the aforesaid problem, in Japanese Patent Application No. 8-245346 that was not yet laid open to the public on the priority date of the application concerned, we have proposed a transflective type display device that employs a reflective polarizer, which is an example of a polarized light separator for reflecting the light of a linearly polarized light component in a predetermined direction and for transmitting the light of a linearly polarized light component in the direction orthogonal thereto, in place of the lower polarizer and the transflective plate on the light source side. According to the display device, the reflection efficiency is enhanced by the polarized light separator, enabling brighter display to be achieved. Further, display devices employing reflective polarizers have been disclosed in Published Japanese Translations of Japanese Unexamined Patent Publication No. 9-506985 (Published International Application: WO/95/17692) and Published International Application No. WO/95/27819.
The transflective type display device employing the reflective polarizer that we have proposed in Japanese Patent Application No. 8-245346 will be described in conjunction with FIG. 1.
In FIG. 1, the display device is equipped with an upper polarizer 301, an upper glass substrate 302, a lower glass substrate 304, a reflective polarizer 306, a semi-transmissive light absorption layer 307, and a light source 308. The display device is further equipped with a TN liquid crystal layer sandwiched between the upper glass substrate 302 and the lower glass substrate 304, the TN liquid crystal layer including a voltage non-applied region 605 and a voltage applied region 606.
First, the white display and the black display in the reflective display mode will be discussed. A light that comes from outside the display device and that is indicated on an optical path 601 is converted by the upper polarizer 301 into a linearly polarized light parallel to a paper surface, twisted by 90 degrees in its direction of polarization in the voltage non-applied region 605 of the TN liquid crystal layer to become a light of a linearly polarized light component perpendicular to the paper surface, reflected by the reflective polarizer 306 as the linearly polarized light perpendicular to the paper surface, twisted again in the voltage non-applied region 605 of the TN liquid crystal layer by 90 degrees in its direction of polarization and becomes a light of the linearly polarized light component parallel to the paper surface, and escapes through the upper polarizer 301. Hence, when no voltage is applied to the TN liquid crystal layer, the white display is provided. Thus, the light in the white display is the light reflected by the reflective polarizer 306 that selectively reflects most of the light of the linearly polarized light transmitted through the upper polarizer 301; hence, the display is brighter than the one provided by the conventional display device (refer to FIG. 33) which employs the transflective plate that merely partially reflects the light transmitted through the polarizer as previously discussed. In contrast to this, a light indicated on an optical path 603 is converted on the upper polarizer 301 into a linearly polarized light parallel to the paper surface, transmitted as the linearly polarized light parallel to the paper surface without being changed in the direction of polarization in the voltage applied region 606 of the TN liquid crystal layer, further transmitted without being changed in the direction of polarization on the reflective polarizer 306, then absorbed by the semi-transmissive light absorption layer 307, thus providing the black display.
The white display and the black display in the transmissive display mode will now be discussed. A light from the light source 308 that is indicated on an optical path 602 is transmitted through an opening provided in the semi-transmissive light absorption layer 307, converted by the reflective polarizer 306 into a linearly polarized light parallel to the paper surface (i.e. the polarized light component perpendicular to the paper surface is reflected by the bottom surface of the reflective polarizer 306 and absorbed by the semi-transmissive light absorption layer 307), twisted by 90 degrees in the direction of polarization in the voltage non-applied region 605 of the TN liquid crystal layer so as to become a linearly polarized light perpendicular to the paper surface, and absorbed by the upper polarizer 301, thus providing the black display. As opposed to this, a light from the light source 308 that is indicated on an optical path 604 is transmitted through the opening provided in the semi-transmissive light absorption layer 307, converted by the reflective polarizer 306 into a linearly polarized light parallel to the paper surface, and transmitted through the upper polarizer plate 301 as the linearly polarized light parallel to the paper surface without changing the direction of polarization even in the voltage applied region 606 of the TN liquid crystal layer, thus providing the white display.
Thus, the transflective type display device that we have proposed in Japanese Patent Application No. 8-245346 and that employs the reflective polarizer as the polarized light separator is able to provide brighter reflective display by external light than that available with the conventional art mainly in a bright place and also to provide transmissive display dependent upon the light from a light source mainly in a dark place.
As discussed in conjunction with FIG. 1, however, the transflective type display device employing the reflective polarizer as the polarized light separator uses the light reflected by the reflective polarizer for performing display in the transmissive display mode, while it uses the light transmitted through the reflective polarizer for performing display in the reflective display mode. Therefore, the portions where a voltage is applied to the liquid crystal panel (where the direction of polarization is not twisted in the TN liquid crystal) present the white display known as xe2x80x9cnegative displayxe2x80x9d in the transmissive display mode, whereas the portions where no voltage is applied to the liquid crystal panel (where the direction of polarization is twisted 90 degrees in the TN liquid crystal) present the white display known as xe2x80x9cpositive displayxe2x80x9d In the reflective display mode. In other words, in the reflective display mode, the white and black display is reversed in comparison with the transmissive display mode. Thus, the display device that we have proposed in Japanese Patent Application No. 8-245346 has been posing a problem in which xe2x80x9cpositive-negative inversionxe2x80x9d takes place if the same liquid crystal applied voltage is provided in the transmissive display mode and the reflective display mode.
Specifically, the positive-negative inversion display is relatively suited for the black-and-white display or dichroic display for primarily displaying characters or numerals because the difference is sufficiently small to enable the recognition of characters or numerals in the positive display and the negative display; it is not suited, however, for full-color display because the displayed colors in the negative display mode are far from actual colors.
Further, the external light serving as the display light passes twice through the optical elements including a polarizer, a liquid crystal panel, a reflective polarizer, and a color filter or the like in a round trip in the reflective display mode, while the light from a light source serving as the display light passes through the above optical elements only once in the transmissive display mode. This generally causes some difference in the light intensity on a display screen. For this reason, there has been another problem in that achieving stable display luminance in both transmissive display mode and the reflective display mode, that is, achieving approximately the same display brightness in both modes, is even more difficult than simply solving the problem of the positive-negative inversion.
The present invention has been made with a view toward solving the aforesaid problems, and it is an object thereof to provide: a transflective type display device which utilizes a variable transmission polarization axis optical element such as a liquid crystal, which does not develop positive-negative inversion upon the switching between reflective display mode dependent on external light and transmissive display mode dependent upon a light source turned ON, which is capable of providing approximately the same display luminance in the reflective display mode and the transmissive display mode, and which is also capable of providing bright display; a driving method for the same; and electronic equipment employing the same.
To this end, according to the present invention, there is provided a display device equipped with: variable transmission polarization axis means capable of varying a transmission polarization axis; first polarized light separating means which is disposed on one side of the variable transmission polarization axis means and which permits a light of a linearly polarized light component in a first direction to be transmitted while it reflects or absorbs a light of a linearly polarized light component in a predetermined direction different from the first direction; second polarized light separating means which is disposed on the other side of the variable transmission polarization axis means and which permits a light of a linearly polarized light component in a second direction to be transmitted while it reflects a light of a linearly polarized light component in a predetermined direction different from the second direction; a light source which is disposed on the opposite side from the variable transmission polarization axis means relative to the second polarized light separating means and which emits light to the variable transmission polarization axis means via the second polarized light separating means; ON/OFF controlling means for controlling the turning ON/OFF of the light source; driving means for driving the variable transmission polarization axis means according to image data to change the transmission polarization axis; and driving characteristic switching means for switching the changing characteristic of the transmission polarization axis with respect to the image data in the driving means in accordance with the ON/OFF of the light source.
In the display device in accordance with the present invention, when the reflective display is effected by utilizing external light, the light source is placed in an OFF state by the ON/OFF controlling means, and external light is entered from the first polarized light separating means. The first polarized light separating means transmits the light of the linearly polarized light components in the first direction among the incident external light to the variable transmission polarization axis means side. And the first polarized light separating means reflects or absorbs linearly polarized light components in the predetermined direction different from the first direction (e.g. a direction orthogonal or nearly orthogonal to the first direction). Then, the second polarized light separating means transmits the light of the linearly polarized light components in the second direction among the light entered via the first polarized light separating means and the variable transmission polarization axis means to the side opposite from the variable transmission polarization axis means and reflects the light of linearly polarized light components in the predetermined direction different from the second direction (e.g. a direction orthogonal or nearly orthogonal to the second direction). The light transmitted through the second polarized light separating means is reflected or diffused at the light source portion that is in an OFF state. On the other hand, the light reflected by the second polarized light separating means passes through the variable transmission polarization axis means and the first polarized light separating means in the reverse order from the above order.
As a result, in the case of the reflective display, a first display state and a second display state set forth below are obtained: in a first display state (relatively bright), the light selectively reflected by the second polarized light separating means according to the direction of the transmission axis in the variable transmission polarization axis means exits the first polarized light separating means side via the variable transmission polarization axis means, and in a second display state (relatively dark), the light transmitted through the second polarized light separating means does not exit the first polarized light separating means side because it is primarily absorbed or diffused. A halftone display can be obtained by appropriately adjusting the polarization axis of the light between the first and second polarized light separating means by the variable transmission polarization axis means and by adjusting the intensity of the light finally exiting the first polarized light separating means. At this time, regarding the brightness in the reflective display, the polarization and separation is carried out by the reflection of light rather than the absorption of light and the reflected linearly polarized light components are utilized as display light; hence, brighter reflective display can be obtained in comparison with the conventional case where the polarizer is employed as the second polarized light separating means.
On the other hand, in the case where the transmissive display uses a light source, the light source is placed in the ON state by the ON/OFF controlling means, and the light of the light source is applied from the light source to the second polarized light separating means. The second polarized light separating means transmits the light of the linearly polarized light components in the second direction among the incident light of the light source to the variable transmission polarization axis means side, while it reflects the light of the linearly polarized light component in the predetermined direction different from the second direction. Further, the first polarized light separating means transmits the light of the linearly polarized light component in the first direction among the light entered via the second polarized light separating means and the variable transmission polarization axis means to the opposite side from the variable transmission polarization axis means, i.e., to the display screen side. And the first polarized light separating means reflects the linearly polarized light component in the predetermined direction different from the first direction.
As a result, in the case of the transmissive display, a third display state and a fourth display state as set forth below are obtained: in the third display state (relatively bright), the light selectively transmitted through the second polarized light separating means according to the direction of the transmission axis in the variable transmission polarization axis means exits the first polarized light separating means side, and in the fourth display state (relatively dark), the light emitted from the light source is reflected by the first polarized light separating means. A halftone display can be obtained by appropriately adjusting the polarization axis of the light between the first and second polarized light separating means by the variable transmission polarization axis means and by adjusting the intensity of the light finally exiting the first polarized light separating means.
Thus, when effecting the reflective display and the transmissive display, the variable transmission polarization axis means is driven by the driving means in accordance with image data to change the transmission polarization axis; hence, an image based on the image data according to the first or second display state in the reflective display mode or the third or fourth display state in transmissive display mode is shown on the display device. In this case, if the same changing characteristic of the transmission polarization axis relative to image data in the driving means were set for both reflective display and transmissive display, then the area to be placed in the first display state (bright) in the reflective display mode would be placed in the fourth display state (dark) in the transmissive display mode, while the area to be placed in the second display state (dark) in the reflective display mode would be placed in the third display state (bright) in the transmissive display mode for the same image data as in the case of the device disclosed in Japanese Patent Application No. 8-245346 previously discussed. This means that the positive-negative inversion takes place upon the switching between the reflective display mode and the transmissive display mode.
The changing characteristic of the transmission polarization axis relative to image data means how to change the transmission polarization axis in relation to the changes in image data. For instance, there are such changing characteristics as the one wherein, if image data is binary data indicative of white or black, then the transmission polarization axis is changed such that the direction of polarization is twisted 90 degrees , 270 degrees, or other degrees for the image data indicative of white, while the direction of polarization is not twisted for the image data indicative of black. Conversely, in another changing characteristic, the transmission polarization axis is changed such that the direction of polarization is not twisted for the image data indicative of black, while the direction of polarization is twisted for the image data indicative of white. For the image data indicative of multiple-step gray scale from white to black, there is available a changing characteristic wherein the transmission polarization axis is changed such that the twisting in the direction of polarization is gradually decreased from white toward black, or another changing characteristic wherein the transmission polarization axis is changed to gradually increase the twisting of the direction of polarization from white toward black.
According to the present invention, when the turning ON and OFF of the light source is controlled by the ON/OFF controlling means, the changing characteristic of the transmission polarization axis for image data in the driving means as described above is switched by the driving characteristic switching means in accordance with the ON or OFF of the light source.
Thus, by setting beforehand the changing characteristic for the reflective display mode and the changing characteristic for the transmissive display mode, respectively, that do not develop the positive-negative inversion as the changing characteristics of the transmission polarization axis for image data, it becomes possible to avoid the occurrence of the positive-negative inversion upon the switching between the reflective display mode and the transmissive display mode by switching between these changing characteristics in accordance with the foregoing ON and OFF in performing actual display.
Further, by switching the changing characteristic of the transmission polarization axis, it becomes also possible to compensate for the difference in the intensity of light between the reflective display mode and the transmissive display mode attributable to the difference in optical path between the external light serving as the display light for the reflective display mode and the light source light serving as the display light for the transmissive display mode. This means that it is possible to provide approximately the same intensity of light in both the reflective display mode and the transmissive display mode. Especially if image data is n-value (n: an integer of 3 or more) data indicative of multiple-step gray scale rather than black-and-white binary data, by setting in advance the changing characteristics for the reflective display mode and the transmissive display mode, respectively so that the intensity of light of the display light (display luminance) in the reflective display mode and the intensity of light of the display light (display luminance) in the transmissive display mode are the same or nearly the same for all steps of gray scale, it becomes possible to obtain the same or nearly the same intensity of light for all steps of gray scale between the reflective display mode and the transmissive display mode by switching between these changing characteristics according to the ON and OFF in performing actual display. To be more specific, by setting the reflectance at each step of gray scale in the reflective display mode at the same or nearly the same intensity of light (display luminance) at each step of gray scale in accordance with the transmittance for each step of gray scale in the transmissive display mode, it becomes possible to set the same or nearly the same intensity of light for all halftone steps in addition to white and black in both transmissive display mode and the reflective display mode. Hence, even in the case of multiple-step gray scale or color image data, high-fidelity, high-definition image display can be achieved. For example, natural, smooth reflective display and transmissive display can be accomplished even for natural images or full-color images.
As set forth above, the display device in accordance with the present invention is able to avoid the positive-negative inversion upon the switching between the reflective display mode and the transmissive display mode and to achieve nearly the same intensity of light (display luminance) in both reflective display mode and transmissive display mode, thus providing bright, high-definition image display. Various specific forms of the method for switching the changing characteristic of the transmission polarization axis in relation to image data are possible, some of them being shown below.
In a preferred form of the display device in accordance with the present invention, the variable transmission polarization axis means is constituted by a liquid crystal panel having a liquid crystal sandwiched between a pair of substrates, and the driving means applies a driving voltage based on the image data to the liquid crystal. In other words, the display device is configured as a liquid crystal device.
With this arrangement, the driving means applies a drive voltage in accordance with image data to the liquid crystal to change the transmission polarization axis in the liquid crystal panel. At this time, when the ON/OFF of the light source is controlled by the ON/OFF controlling means, the changing characteristic of the transmission polarization axis in relation to the image data in the driving means is switched by the driving characteristic switching means in accordance with ON/OFF of the light source. Therefore, in this liquid crystal device, the positive and negative are not inverted when switching between the reflective display mode and the transmissive display mode, and approximately the same intensity of light can be obtained for both reflective display mode and transmissive display mode, enabling bright display to be accomplished.
In this form, the liquid crystal may be composed of any one of TN (Twisted Nematic) liquid crystal, STN (Super-Twisted Nematic) liquid crystal, F-STN (Film compensated Super-Twisted Nematic) liquid crystal, and ECB (Electrically Controlled Birefringence) liquid crystal. With this arrangement, bright, high-definition reflective display can be performed relatively easily without positive-negative inversion. The STN liquid crystal element includes an STN liquid crystal element that employs an optical anisotropic substance for color compensation. Use of an ECB liquid crystal element or other type of liquid crystal element having birefringence effect makes it possible to change the color development from a light source.
In another preferred form of the invention wherein the variable transmission polarization axis means is composed of a liquid crystal panel, the driving characteristic switching means may switch the driving voltage in synchronization with an ON/OFF control signal instructing the ON/OFF controlling means to turn ON/OFF the light source.
With this arrangement, the turning ON/OFF of the light source can be controlled by the ON/OFF controlling means in accordance with the ON/OFF control signal directing the turning ON/OFF of the light source. And in synchronization with the ON/OFF control signal, the driving voltage is switched by the driving characteristic switching means thereby to switch the changing characteristic of the transmission polarization axis of the liquid crystal panel in relation to image data. As a result, the moment the switching is made between the reflective display mode and the transmissive display mode, the driving voltage of the liquid crystal panel is accordingly switched, so that there is hardly or no time for the positive-negative inversion to occur. In other words, a reliable, convenient function for preventing the positive-negative inversion can be realized. As an alternative, the driving voltage of the liquid crystal panel may be switched in synchronization with an ON/OFF detection signal generated by optically or electrically detecting ON/OFF in place of using such an ON/OFF control signal.
In a further preferred form wherein the driving voltage is switched in synchronization with the ON/OFF control signal, the liquid crystal panel is is constituted by a dot matrix liquid crystal panel which is provided with a plurality of data signal lines and a plurality of scanning signal lines and which is able to change the transmission polarization axis by driving the liquid crystal in each driving region formed for each intersection of the plurality of data signal lines and the plurality of scanning signal lines, and the driving characteristic switching means may switch the potentials of data signals supplied to the data signal lines according to the image data in synchronization with the ON/OFF control signal.
With this arrangement, the potentials of the data signals are switched by the driving characteristic switching means in synchronization with the ON/OFF control signal thereby to switch the changing characteristic of the transmission polarization axis of the liquid crystal panel in relation to image data. As a result, also in the dot matrix liquid crystal panel, the moment the switching between the reflective display mode and the transmissive display mode is done, the driving voltage is switched, preventing the inversion between positive and negative from taking place.
In the form wherein the potential of a data signal is switched, the driving means may include a data signal potential supplying means that supplies the potential of the data signal to the liquid crystal panel, and the driving characteristic switching means may include, in a stage preceding the data signal potential supplying means, a data signal converting means for switching the data signal, which is supplied to the data signal potential supplying means in accordance with the image data, to a data signal corresponding to positive display and a data signal corresponding to negative display in synchronization with the ON/OFF control signal.
With this arrangement, in the stage preceding the data signal potential supplying means, data signals can be switched by the data signal converting means into data signals corresponding to positive display and data signals corresponding to negative display in synchronization with the ON/OFF control signal. Hence, the changing characteristic of the transmission polarization axis of the liquid crystal panel can be changed according to image data in the stage of data signals by changing the contents of the data, thus making it possible to prevent the positive-negative inversion relatively easily and reliably.
In the form wherein the data signals are switched by the data signal converting means, the data signal converting means may include an inverting means for inverting the data signal in synchronization with the ON/OFF control signal.
With this arrangement, the changing characteristic of the transmission polarization axis of the liquid crystal panel can be switched according to image data by inverting the data signal by the inverting means; therefore, the positive-negative inversion can be prevented extremely easily and reliably.
In another preferred form of the display device in accordance with the present invention, the second polarized light separating means is composed of a reflective polarizer that allows light of a linearly polarized light component in the second direction to be transmitted and reflects light of a linearly polarized light component in the direction orthogonal to the second direction.
With this arrangement, the reflective polarizer allows the linearly polarized light component in the second direction in the incident light to be transmitted as the linearly polarized light component in the second direction. And the reflective polarizer reflects the linearly polarized light component in the direction orthogonal to the second direction as the linearly polarized light component in the direction orthogonal to the second direction. Hence, display can be performed according to the light transmitted through the reflective polarizer, and the utilization factor of light can be improved over the case wherein light is absorbed to be subjected to polarization and separation by a polarizer as in the conventional example, thus achieving brighter display especially in the reflective display mode.
In this form, the reflective polarizer may be constructed of a laminate composed of first layers having birefringence and second layers that have a refractive index substantially equal to one of a plurality of refractive indexes of the first layers and that has no birefringence, the first and second layers being stacked alternately.
In the reflective polarizer having such a configuration, the linearly polarized light components in the second direction among the light entered from the depositing direction relative to one principal plane of the reflective polarizer are transmitted to the other principal plane side on the opposite side as the linearly polarized light components in the second direction. And the light of the linearly polarized light components in the direction orthogonal to the second direction are reflected as the light of the linearly polarized light components in the direction orthogonal to the second direction. Further, the light of the linearly polarized light components in the second direction among the light entered from the depositing direction relative to the other principal plane of the reflective polarizer are transmitted to one principal plane side on the opposite side as the linearly polarized light components in the second direction. And the light of the linearly polarized light components in the direction orthogonal to the second direction are reflected as the light of the linearly polarized light components in the direction orthogonal to the second direction.
In a further preferred form of the display device in accordance with the present invention, the second polarized light separating means allows the linearly polarized light components in the second direction to be transmitted with respect to the light in the almost entire wavelength range of a visible light region and it reflects the light of the linearly polarized light components in the direction orthogonal to the second direction.
According to this form, in the reflective display mode, two display states are obtained for the external light in the almost entire wavelength range of the visible light region in accordance with the direction of the transmission polarization axis in the variable transmission polarization axis means, display based on transparent reflection or white reflection being obtained in one of the two display states. On the other hand, when a white light source is employed in the transmissive display mode, two display states are obtained for light of a light source in the almost entire wavelength range of the visible light region in accordance with the direction of the transmission polarization axis in the variable transmission polarization axis means, display based on transparent reflection or white reflection being obtained in one of the two display states.
In a further preferred form of the display device in accordance with the present invention, the first polarized light separating means is composed of a polarizer that allows the light of the linearly polarized light components in the first direction to be transmitted and that absorbs the light of the linearly polarized light components in the direction orthogonal to the first direction.
With this arrangement, the polarizer allows the linearly polarized light components in the first direction among the incident light to be transmitted as the linearly polarized light components in the first direction, while it absorbs the linearly polarized light components in the direction orthogonal to the first direction. Hence, the display can be carried out according to the light transmitted through the polarizer, enabling the reflection of external light on a display surface to be reduced.
Another preferred form of the display device in accordance with the present invention is further provided with a semi-transmissive light absorption layer located between the second polarized light separating means and the light source.
With this arrangement, in the reflective display mode, the light transmitted through the second polarized light separating means via the variable transmission polarization axis means is partly absorbed by the semi-transmissive light absorption layer, and the light that has been transmitted through the semi-transmissive light absorption layer is reflected or diffused by the surface of a light source in an OFF state before being further absorbed by the semi-transmissive light absorption layer; therefore, the light is hardly or never emitted from the first polarized light separating means via the second polarized light separating means and the variable transmission polarization axis means. This permits darker display, leading to improved contrast. Further, in the transmissive display. mode, the semi-transmissive light absorption layer allows the light from a light source in the ON state to be partly transmitted, thus enabling the transmissive display.
In this form, the transmittance of the semi-transmissive light absorption layer may range from 5% to 80%.
With this arrangement, proper balance between brightness and contrast can be accomplished in the reflective display mode and the transmissive display mode.
Yet another preferred form of the display device in accordance with the present invention is further provided with a polarizing means that has its transmission axis nearly aligned with the second direction and that is located between the second polarized light separating means and the light source.
According to this form, in the transmissive display mode, the light of the linearly polarized light components in a predetermined direction, which is different from the second direction (e.g. a direction orthogonal to the second direction), among the light from the light source are transmitted through the polarizing means and the second polarized light separating means. At this time, the polarizing means works to make up for the degree of polarization in the second polarized light separating means, so that the contrast in the transmissive display mode is improved. Alternatively, an inexpensive second polarized light separating means that has a relatively lower degree of polarization may be used.
In the form wherein the potentials of data signals can be switched, a nonlinear element for each intersection may be further provided.
With this arrangement, by using a nonlinear element such as TFT or TFD, a display device equipped with a large dot matrix liquid crystal panel of an active matrix driving system that permits high-definition image display can be realized.
In still another form of the display device in accordance with the present invention, a transmissive light diffusion layer is further provided between the light source and the second polarized light separating means.
With this arrangement, the light that is transmitted through the variable transmission polarization axis means and the first polarized light separating means and that is emitted as display light enables the display that is not in a mirror surface state (in a paper state). The light diffusion layer may be disposed, for example, between the first polarized light separating means and the variable transmission polarization axis means or between the variable transmission polarization axis means and the second polarized light separating means.
In the foregoing form wherein the variable transmission polarization axis means is composed of the liquid crystal panel, one of the paired substrates may be further equipped with a color filter.
With this arrangement, a transflective type display device that permits color display such as dichroic display other than black-and-white display or full color display can be achieved.
In the foregoing form wherein the variable transmission polarization axis means is composed of the liquid crystal panel, the liquid crystal panel is formed of a dot matrix liquid crystal panel that is provided with a plurality of data signal lines and a plurality of scanning signal lines and that is able to change the transmission polarization axis by driving the liquid crystal in the respective driving regions formed at the respective intersections of the plurality of data signal lines and the plurality of scanning signal lines. The driving means includes a scanning signal supplying means for supplying the scanning signals and a data signal supplying means for supplying the data signals. The driving voltage applied to the liquid crystal corresponding to each driving region mentioned above may be made different by the driving voltage switching means, depending on whether the light source is OFF or ON by controlling the voltage of at least one of the scanning signals and the data signals respectively supplied by the scanning signal supplying means and the data signal supplying means.
With this arrangement, by setting in advance the driving voltage so that the reflectance in each gray scale is the same or approximately the same in each driving region (dot) in both transmissive display mode and reflective display mode, the intensity of light over an entire screen can be made the same or approximately the same in both transmissive display mode and reflective display mode by selectively applying either one of these different driving voltages to the liquid crystal of each driving region depending on whether the light source is ON or OFF when effecting actual transmissive display and reflective display.
In the preferred form, the driving voltage switching means may further include a scanning signal potential controlling means that makes the potential of the scanning signal supplied by the scanning signal supplying means when the light source is OFF different from the potential of the scanning signal supplied when the light source is ON.
With this arrangement, by setting in advance the potential of the scanning signal for the reflective display and the potential of the scanning signal for the transmissive display so that the intensity of light (display luminance) of each gray scale is the same or approximately the same in both transmissive display mode and reflective display mode, the intensity of light can be made the same or approximately the same in both transmissive display mode and reflective display mode by selectively supplying either of the scanning signals of the different potentials depending on whether the light source is ON or OFF in performing actual transmissive display and reflective display.
Further in this form, the scanning signal potential controlling means may include a first common potential output unit that outputs a predetermined potential and a first converted potential output unit that outputs a potential based on an ON/OFF control signal instructing the ON/OFF controlling means to turn ON/OFF the light source, and may be configured such that the scanning signal potential controlling means outputs the sum of the potentials supplied from the first common potential output unit and the first converted potential output unit to the scanning signal supplying means.
With this arrangement, it is possible to reduce power consumption and simplify the circuit configuration and to reliably provide the same or approximately the same intensity of light in the transmissive display mode and the reflective display mode.
In the aforesaid form wherein the voltage of at least either the scanning signal or the data signal is controlled, the driving voltage switching means may be constructed to include a data signal potential controlling means that makes setting so that the potential of the data signal supplied by the data signal supplying means when the light source is OFF is different from the potential of the data signal supplied when the light source is ON.
With this arrangement, by setting in advance the potential of the data signal for the reflective display and the potential of the data signal for the transmissive display so that the intensity of light (display luminance) of each gray scale step is the same or approximately the same in both transmissive display mode and reflective display mode, the intensity of light can be made the same or approximately the same in both transmissive display mode and reflective display mode by selectively supplying either one of the data signals of the different potentials depending on whether the light source is ON or OFF when performing actual transmissive display and reflective display.
Further in this form, the data signal potential controlling means may include an image signal repeater that outputs a potential corresponding to the image data, a second common potential output unit that outputs a predetermined potential, and a second converted potential output unit that outputs a potential based on the ON/OFF control signal instructing the ON/OFF controlling means to turn ON/OFF the light source, and may be configured such that the data signal potential controlling means outputs the sum of the potentials output from the image signal repeater, the second common potential output unit, and the second converted potential output unit to the data signal supplying means.
With this arrangement, it is possible to reduce power consumption and simplify the circuit configuration and to reliably provide the same or approximately the same intensity of light in the transmissive display mode and the reflective display mode.
In the foregoing form that includes the data signal potential controlling means, the data signal potential controlling means may include an image signal converting unit that converts an image signal corresponding to the image data and outputs the potential of the converted image signal, a third common potential output unit that outputs a predetermined potential, and a gray scale controlling unit that outputs a potential based on an ON/OFF control signal instructing the ON/OFF controlling means to turn ON/OFF the light source and the gray scale information of the image data. The data signal potential controlling means may be configured such that it outputs the sum of the potentials output from the image signal converting unit, the third common potential output unit, and the gray scale controlling unit to the data signal supplying means.
With this arrangement, it is possible to reduce power consumption and simplify the circuit configuration and to reliably provide the same or approximately the same intensity of light in the transmissive display mode and the reflective display mode.
The aforesaid object of the present invention is fulfilled also by electronic equipment provided with the display device in accordance with the present invention described above.
The electronic equipment in accordance with the present invention is provided with the display devices in accordance with the present invention as set forth above, thus making it possible to realize a variety of electronic equipment such as portable information equipment, a personal computer, a navigation system or the like that are capable of providing bright display without the inversion of positive and negative when switching between the reflective display mode and the transmissive display mode. The electronic equipment in accordance with the present invention may be equipped with any one of the display devices in the various forms set forth above depending on its application.
The foregoing object of the present invention may be fulfilled also by a driving method of a display device provided with variable transmission polarization axis means capable of varying a transmission polarization axis, first polarized light separating means which is disposed on one side of the variable transmission polarization axis means and which permits a light of a linearly polarized light component in a first direction to be transmitted while it reflects or absorbs a light of a linearly polarized light component in a predetermined direction different from the first direction, second polarized light separating means which is disposed on the other side of the variable transmission polarization axis means and which permits a light of a linearly polarized light component in a second direction to be transmitted while it reflects a light of a linearly polarized light component in a predetermined direction different from the second direction, and a light source which is disposed on the opposite side from the variable transmission polarization axis means with respect to the second polarized light separating means and which emits light to the variable transmission polarization axis means via the second polarized light separating means, the driving method of a display device comprising: an ON/OFF controlling step for controlling the turning ON/OFF of the light source; a driving step for driving the variable transmission polarization axis means according to image data to change the transmission polarization axis; and a driving characteristic switching step for switching the changing characteristic of the transmission polarization axis relative to the image data in the driving step in accordance with the ON/OFF of the light source.
The driving method in accordance with the present invention makes it possible to avoid the positive-negative inversion upon the switching between the reflective display mode and the transmissive display mode and to achieve nearly the same intensity of light (luminance of display) in both reflective display mode and transmissive display mode, providing bright, high-definition image display as in the case of the display device in accordance with the present invention described above.
In a preferred form of the driving method of the display device in accordance with the present invention, the variable transmission polarization axis means is constituted by a liquid crystal panel having a liquid crystal sandwiched between a pair of substrates, and the driving step applies a drive voltage based on the image data to the liquid crystal.
With this arrangement, the driving voltage in accordance with image data is applied to the liquid crystal to change the transmission polarization axis in the liquid crystal panel. At this time, when the ON/OFF of the light source is controlled, the changing characteristic of the transmission polarization axis in relation to the image data in the driving step is switched in accordance with ON/OFF of the light source. Therefore, in this liquid crystal device, the positive and negative are not inverted upon the switching between the reflective display mode and the transmissive display mode, and approximately the same intensity of light can be obtained for both reflective display mode and transmissive display mode, enabling bright display to be accomplished.
Further in this form, the driving characteristic switching step may switch the driving voltage in synchronization with an ON/OFF control signal that instructs the turning ON/OFF of the light source in the ON/OFF controlling step.
With this arrangement, the turning ON/OFF of the light source can be controlled by the ON/OFF controlling step in accordance with the ON/OFF control signal directive of the turning ON/OFF of the light source. And in synchronization with the ON/OFF control signal, the driving voltage is switched so as to switch the changing characteristic of the transmission polarization axis of the liquid crystal panel relative to image data. As a result, upon the switching between the reflective display mode and the transmissive display mode, the driving voltage of the liquid crystal panel is accordingly switched, so that there is hardly or no time for the positive-negative inversion to occur.
Further in this form, the liquid crystal panel is composed of a dot matrix liquid crystal panel which is equipped with a plurality of data signal lines and a plurality of scanning signal lines, and in which the transmission polarization axis can be changed by driving the liquid crystals in respective driving regions formed for the respective intersections of the plurality of data signal lines and the plurality of scanning signal lines. The driving characteristic switching step may switch the potentials of data signals supplied to the data signal lines according to the image data in synchronization with the ON/OFF control signal.
With this arrangement, the potentials of the data signals are switched in synchronization with the ON/OFF control signal thereby to switch the changing characteristic of the transmission polarization axis of the liquid crystal panel relative to image data. As a result, also in the dot matrix liquid crystal panel, upon the switching between the reflective display mode and the transmissive display mode, the driving voltage is switched, thus preventing the inversion of positive and negative from taking place.
Further in this form, the driving step may include a data signal potential supplying step that supplies the potential of the data signal to the liquid crystal panel, and the driving characteristic switching step may include, prior to the data signal potential supplying step, a data signal converting step for switching the data signal supplied in accordance with the image data in the data signal potential supplying step to a data signal corresponding to positive display and a data signal corresponding to negative display in synchronization with the ON/OFF control signal.
With this arrangement, data signals can be switched by the data signal converting step into data signals corresponding to positive display and data signals corresponding to negative display in synchronization with the ON/OFF control signal. Hence, the changing characteristic of the transmission polarization axis of the liquid crystal panel can be switched according to image data in the stage of data signals by changing the contents of the data, thus making it possible to prevent the positive-negative inversion relatively easily and reliably.
Further in this form, the data signal converting step may include an inverting step for inverting the data signal in synchronization with the ON/OFF control signal.
With this arrangement, the changing characteristic of the transmission polarization axis of the liquid crystal panel can be switched according to image data by inverting the data signal by the inverting step; therefore, the positive-negative inversion can be prevented extremely easily and reliably.
In the foregoing form wherein the transmission polarization axis means is composed of the liquid crystal panel, the liquid crystal panel is formed of a dot matrix liquid crystal panel that is provided with a plurality of data signal lines and a plurality of scanning signal lines and that is able to change the transmission polarization axis by driving the liquid crystal in the respective driving regions formed at the respective intersections of the plurality of data signal lines and the plurality of scanning signal lines. The driving step includes a scanning signal supplying step for supplying the scanning signals and a data signal supplying step for supplying the data signals. The driving voltage switching step may set the driving voltage applied to the liquid crystal corresponding to each driver region mentioned above so that it is different depending on whether the light source is OFF or ON by controlling the voltage of at least one of the scanning signals and the data signals respectively supplied by the scanning signal supplying step and the data signal supplying step.
With this arrangement, by setting in advance the driving voltage so that the reflectance in each gray scale step is the same or approximately the same in each driving region (dot) in both transmissive display mode and reflective display mode, the intensity of light over an entire screen can be made the same or approximately the same in both transmissive display mode and reflective display mode.
In the preferred form, the driving voltage switching step may further include a scanning signal potential controlling step that makes setting so that the potential of the scanning signal supplied by the scanning signal supplying step when the light source is OFF is different from the potential of the scanning signal supplied when the light source is ON.
With this arrangement, by setting in advance the potential of the scanning signal for the reflective display and the potential of the scanning signal for the transmissive display so that the intensity of light (display luminance) of each gray scale step is the same or approximately the same in both transmissive display mode and reflective display mode, the intensity of light can be made the same or approximately the same in both transmissive display mode and reflective display mode.
In the aforesaid form wherein the voltage of at least either the scanning signal or the data signal is controlled, the driving voltage switching step may include a data signal potential controlling step that makes setting so that the potential of the data signal supplied by the data signal supplying step when the light source is OFF is different from the potential of the data signal supplied when the light source is ON.
With this arrangement, by setting in advance the potential of the data signal for the reflective display and the potential of the data signal for the transmissive display so that the intensity of light (display luminance) of each gray scale step is the same or approximately the same in both transmissive display mode and reflective display mode, the intensity of light can be made the same or approximately the same in both transmissive display mode and reflective display mode.
The display devices in accordance with the present invention described above are able to achieve the display free of the positive-negative inversion upon the switching between the reflection mode and the transmission mode while realizing brighter reflective display even when they are constructed as display devices of any of the publicly known driving systems such as the simple (passive) matrix system or the active matrix system or the segment system that employ TFTs (Thin Film Transistors) or TFDs (Thin Film Diodes).
Furthermore, as the polarized light separating means in accordance with the present invention, other type than the foregoing reflective polarizer may be used, some examples being a combination of a cholesteric liquid crystal layer and a (xc2xc) xcex plate, one adapted for separation into reflected polarized light and transmitted polarized light by making use of Brewster""s angle (page 427 through page 429, S1D 92 D1GEST), one making use of hologram, and the ones disclosed in the international application laid open (published International Applications,: No. WO95/27819 and No. WO95/17692). The diverse types of polarized light separator can be used in place of the reflective polarizer in the same manner in the embodiments to be discussed hereafter.